(a) Technical Field
The present disclosure relates to a separator for a polymer electrolyte membrane fuel cell (PEMFC). More particularly, it relates to a composite separator for a PEMFC and a method for manufacturing the same, in which a graphite foil prepared by compressing expanded graphite is stacked on a carbon fiber-reinforced composite prepreg or a mixed solution prepared by mixing graphite flake and powder with a resin solvent is applied to the cured composite prepreg such that a graphite layer is integrally molded on the outermost end of the separator.
(b) Background Art
In general, a polymer electrolyte membrane fuel cell (PEMFC) is a device that generates electricity with water produced by an electrochemical reaction between hydrogen and oxygen. The PEMFC has various advantages such as high energy efficiency, high current density, high power density, short start-up time, and rapid response to a load change as compared to the other types of fuel cells. For these reasons, it can be used in various applications such as a power source for zero-emission vehicles, an independent power plant, a portable power source, a military power source, etc.
The configuration of a fuel cell stack will be briefly described with reference to FIG. 1 below.
In a typical fuel cell stack, a membrane-electrode assembly (MEA) is positioned in the center of each unit cell of the fuel cell stack. The MEA comprises a solid polymer electrolyte membrane 60, through which hydrogen ions (protons) are transported, and catalyst layers including a cathode 61 and an anode 61, which are coated on both surfaces of the electrolyte membrane 60 to allow hydrogen and oxygen to react with each other.
Moreover, a gas diffusion layer (GDL) 40 and a gasket 41 are sequentially stacked on the outside of the electrolyte membrane 10, i.e., on the surface where the cathode and the anode are positioned. A separator (also called a bipolar plate) 30 including flow fields, through which fuel is supplied and water generated by a reaction is discharged, is positioned on the outside of the GDL 40. And, an end plate 50 for supporting the above-described elements is connected to the outermost end.
Therefore, an oxidation reaction of hydrogen occurs at the anode of a fuel cell to produce hydrogen ions (protons) and electrons, and the produced hydrogen ions and electrons are transmitted to the cathode through the electrolyte membrane and the separator, respectively. At the cathode, the hydrogen ions and electrons transmitted from the anode through the electrolyte membrane and the separator react with oxygen in air to produce water. Here, electrical energy is generated by the flow of the electrons through an external conducting wire due to the transfer of the hydrogen ions.
In the above-described fuel cell stack, the separator separates the respective unit cells of the fuel cell and, at the same time, serves as a current path between the unit cells, and the flow fields formed in the separator serve as paths through which hydrogen and oxygen are supplied and water produced by the reaction is discharged.
Since the water produced by the reaction inhibits the chemical reaction occurring in the electrolyte membrane of the fuel cell, the water should be rapidly discharged to the outside, and therefore the separator material may have high surface energy such that the water is rapidly spread on the surface of the separator (hydrophilicity) or may have low surface energy such that the water rolls down the surface of the separator (hydrophobicity).
Therefore, it is necessary to minimize the electrical contact resistance between the separators and, at the same time, maximize the hydrophilicity or hydrophobicity of the flow fields to allow the product water to smoothly flow.
Conventionally, the separator is formed of graphite, thin stainless steel, or a composite material in which expanded carbon particles or graphite particles are mixed with a polymer matrix. However, recently, an attempt to prepare a composite separator using continuous carbon fibers has been made.
Although the electrical resistance of stainless steel is significantly lower than that of graphite (see Table 1), the electrical contact resistance of graphite is measured lower than that of stainless steel, since the electrical contact resistance is related to the contact area and pressure and the hardness of the material.
Moreover, although the graphite satisfies the conditions of the separator in its electrical and chemical requirements, it is vulnerable to impact and is hard to process. Therefore, a research aimed at developing a continuous carbon fiber composite separator which satisfies the electrical, chemical, and mechanical requirements has continued to progress. Further, in order to reduce the contact resistance between the unit cells, and to efficiently discharge water produced by the reaction, a method for manufacturing a separator capable of controlling the surface energy of the continuous carbon fiber composite separator is required.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.